1. Field of the Invention
The present invention relates to the field of fabrication of integrated circuits, and, more particularly, to electrochemically treating substrates during various steps of production to deposit and/or remove a metal on/from the substrate.
2. Description of the Related Art
The materials used in multilevel interconnect technology of integrated circuits (IC) are thin films of conductors and thin films of insulators. To manufacture conductive thin films, aluminum and aluminum alloys have been widely used in combination with silicon dioxide (SiO2) as an insulator. To further improve device performance, in view of signal propagation delay and power consumption of an IC, copper, possibly in combination with a low-k dielectric material, is nowadays increasingly replacing aluminum and silicon dioxide. Furthermore, the use of the copper technology results in a reduction of the number of necessary metallization levels. In manufacturing multilevel interconnection systems, plating, in the form of electroplating and electroless plating, and the reverse process, also referred to as an electropolishing process, have become widely-used metal deposition/removing techniques.
To obtain a required quality of the metal layers, typically a variety of chemicals are used in the electrochemical metal plating process. In many electrolytes used for plating a metal on a substrate, an inorganic acid is used as the main ingredient for the plating solution. Sulfuric acid or phosphoric acid is widely used in a variety of concentrations. Sulfuric acid and phosphoric acid are known to etch copper, irrespective of the concentration with which the sulfuric acid and phosphoric acid are provided. The etch rate is further increased when oxygen is readily available at the metal areas formed on semiconductor substrates, as is the case in a conventional plating process due to the oxygen contained in the ambient air.
Owing to the presence of oxygen, sulfur dioxide (that may be present in the ambient in minute amounts) and water (contained in the water-diluted acids), a significant degree of oxidation and discoloration of the metal formed on the semiconductor substrates may occur. This situation is even exacerbated during subsequent transport, store, rinse and cleaning actions, all of which are performed under wet conditions, i.e., under conditions that promote oxidation and discoloration of copper. The same is true for copper-containing substrates that are subjected to an electropolishing process that is quite similar to electroplating, wherein, however, the flow of the ions is reversed.
Since copper is increasingly used in semiconductor production and, as explained above, since exposed copper surfaces tend to readily react with oxygen to form corrosion and discoloration, this corrosion and discoloration may, in turn, tend to compromise the quality of the resulting surface and adversely affect subsequent process steps. In view of the foregoing, it is clear that the plating and electropolishing of copper on semiconductor substrates is critical for the reliability of the completed integrated circuit.
Therefore, a need exists for an apparatus and a method to form and treat metal layers, especially copper layers, without unduly deteriorating the surface quality.